Next generation nonvolatile memories receiving attention in response to a demand for a new medium for information storage are ferromagnetic random access memory (FeRAM), magnetic random access memory (MRAM), resistive random access memory (ReRAM), phase-change random access memory (PRAM), etc. Each of these memories has unique advantages, and there is actively ongoing research into suitable use for each.
Of these, utilizing a quantum mechanical effect called magnetoresistance, MRAM is a large capacity nonvolatile memory with high energy efficiency, high density, and high responsiveness characteristics that can replace the currently widely used DRAM.
Giant magneto resistance (GMR) and tunneling magneto resistance (TMR) are two widely known magnetoresistance effects.
An device utilizing the GMR effect stores information by utilizing a resistance change effect of a conductive layer sandwiched between two ferromagnetic layers, with the resistance change depending on spin polarization of the ferromagnetic layers above and below. However, in a GMR device, because magnetoresistance ratio representing a rate of resistance change is low, at about 10%, information storage detection signal is small and, thus, obtaining a sufficient detection margin is the biggest challenge for realization of an MRAM device.
Meanwhile, well known device utilizing the TMR effect is a magnetic tunnel junction (MTJ) device that utilizes resistance change that depends on a magnetic tunnel junction effect.
The MTJ device has a layer structure of ferromagnetic layer/insulating layer/ferromagnetic layer. In an MTJ device, when spin directions of the layer above and below are identical, tunneling probability between the two ferromagnetic layers with an interposed insulating layer is maximized, and minimum resistance thus results. On the other hand, when spin directions of the layers above and below are opposite each other, tunneling probability is minimized, and resistance is maximized.
To realize the two spin states, magnetization direction of one of the ferromagnetic layers (a magnetic film) is fixed not to be influenced by external magnetization. A ferromagnetic layer with a fixed magnetization direction is generally called a fixed layer or a pinned layer.
Magnetization direction of the other ferromagnetic layer (a magnetic film) can be the same as or opposite the magnetization direction of the fixed layer, depending on the direction of an applied magnetic field. The ferromagnetic layer in this case is generally called a free layer and plays the role of storing information.
An MTJ device with a magnetoresistance (MR) ratio due to resistance change exceeding 50% is currently achieved and used mainly used in MRAM development.
Meanwhile, of the MTJ devices, an MTJ device utilizing a perpendicular magnetic an isotropic material is gathering attention.
In particular, research is actively ongoing for applying an MTJ device utilizing the perpendicular magnetic anisotropic material to perpendicular spin-transfer torque magnetoresistance memory (STT-MRAM), etc.
Spin-transfer torque (STT) recording refers to a method of inducing magnetization reversal by direct application of a current to a magnetic tunnel junction, rather than using an external magnetic field. The STT recording method does not require separate external wiring, thus being characteristically advantageous for high density integration.
Currently, a representative magnetic layer for an MTJ device is a layer of CoFeB material, and a structure of Ta seed layer/CoFeB magnetic layer/MgO tunneling barrier layer can provide an MTJ device with high MR, when grown with maintaining a body centered cubic (BCC) (001) crystal structure.
As a manufacturing method for the MTJ device, a method of establishing BCC (001) growth of a magnetic layer is published, in which a Ta based seed layer is deposited amorphously, after which a CoFeB magnetic layer with doped boron is again grown amorphously, MgO material is deposited on the magnetic layer in a BCC (001) crystal structure, and, after depositing all of the layers, performing a post-thermal processing thereof.
Also, it is known that BCC (001) growth of an existing amorphous CoFeB material is assisted by a portion of the boron escaping from the CoFeB material after a thermal processing.
However, when a post-thermal processing is performed, boron material escaping due to a diffusion effect combines with the MgO layer in an MTJ device to form MgBO and influences MR or causes problems with the Ta seed layer used.
Also, although, after a post-thermal processing, a portion of the Ta seed layer used in a Ta/CoFeB/MgO structure is used as a boron absorbing layer, diffusion of the layer itself occurring due to heat causes a problem in terms of lowered MR magnitude and thermal stability.
Further, because the CoFeB magnetic layer fundamentally has a large damping constant, current density Jc reduction, which is essential for device development, has a limit.
Accordingly, there is a demand for essential development of a new ferromagnetic material with a smaller damping constant for reducing current density Jc, and, for this reason, there is foremost a need for development of a seed layer material with a novel and different structure for suitably growing a new magnetic material.